Various zoom lenses have been developed for use in video cameras that are provided with a solid-state image sensor, such as a CCD (Charge Coupled Device). For example, a zoom lens that has a four lens group or a five lens group construction is known wherein the second lens group and the third lens group, in order from the object side, are moved to perform a change of magnification during zooming. In zoom lenses having such a construction, focusing techniques can be broadly classified into two categories, namely, those wherein the first lens group moves during focusing, and those wherein the first lens group is stationary and one or more of the other lens groups move during focusing. Tile latter category is termed herein a “rear focus zoom lens”.
With a rear locus zoom lens, tile effective diameter of the first lens group can be smaller as compared with other types of zoom lenses wherein the entire first lens group is moved for focusing. Thus, miniaturization of the entire lens system becomes easy. Further, the movement of one or more lens groups that are comparatively small in size and thus lightweight, as compared to the first lens group, enables faster focusing. Thus, a rear focus zoom lens is characterized by the required driving force for moving the lens groups being small while enabling rapid focusing to be performed. In recent years, miniaturization of solid-state image sensors has rapidly progressed and this has enabled the size of video camera bodies to be miniaturized. Therefore, there has been a strong demand for a zoom lens that is both small in size and lightweight for mounting onto a video camera. From this viewpoint, a rear focus zoom lens is advantageous.
The rear focus zoom lenses that are disclosed in Japanese Laid-Open Patent Application H7-151972 and Japanese Patent Publication 2988164 have a five-lens-group construction, and these zoom lenses have adopted a method wherein the second lens group and the third lens group move during zooming. The zoom lens that is disclosed in Japanese Patent Publication 2893119 has a four-lens-group construction, and this zoom lens has adopted a method wherein the second lens group and the third lens group move during zooming. As focusing methods, examples are given wherein part or all of the lenses of the fourth lens group move during focusing. As a movement lens group for focusing, an example wherein the fourth lens group is comprised of a two-lens-element construction using lens elements that are separated by air is disclosed.
However, each of the above-discussed zoom lenses has a zoom ratio of approximately 10, and the angle of view at the wide-angle end remains about 60 degrees, so the development of a zoom lens having a higher zoom ratio (for example, approximately 14) and a wider angle of view (for example, approximately 70 degrees) has been desired. Further, in recent years, in order to achieve higher picture quality, an image pick-up device with a large number of pixels has been developed. With these developments, a higher performance zoom lens has also been demanded. Therefore, the development of a zoom lens having a smaller size, a wider angle of view and a higher zoom ratio, while maintaining high optical performance, has been desired.